1. Field of the Invention
The present invention relates ton heating printed circuit boards, and more particularly to convection heating such printed circuit boards uniformly and quickly.
2. Background Information
Large printed circuit boards (PCBs) have multiple layers where some layers bear narrow etched conductive runs and other layers might have large ground and/or power planes. Such PCBs and methods for mounting the components and soldering the boards are well known in the art and only a brief review is present herein. The conductive runs on the layers are interconnected by plated through holes, called vias, which traverse the thickness of the PCB making the designed electrical connections between the various layers. Usually one external side only has etched runs while the opposite external side is bears components, although it is known to have components on both sides. The electrical connections through the populated board are soldered, typically, on a wave solder machine or by re-flow techniques. Such techniques are well known in the art. But, when such boards are large and carry a wide assortment of components they are difficult to solder reliably. Later it is difficult to rework defective solder joints on such boards. Part of the problem of soldering and/or re-soldering such boards stems from temperature differentials on the board that stress the etched runs and vias causing small cracks or even full breaks. Such cracks and/or breaks produce defective boards. The problem is insidious when cracks occur in etched runs that may not be detected at inspection, because such cracks will often cause field failures that are expensive to repair and, even more importantly, which customers experience, thereby damaging the product and its maker""s reputation.
Maintaining even temperatures over the extended area of these large PCBs is difficult, and is made more difficult by the uneven mass distribution of board mounted components and large areas of plated ground planes. These differences cause differing final temperatures in areas of the board and different times to stabilize at a temperature. But, temperature differentials and the resulting thermal stress must be minimized to preserve the integrity, reliability and long life of such PCB assemblies.
Practitioners in the field have used infrared heat sources, conduction and convection methods for heating PCBs. Convection using a gas is attractive since a large amount of heat energy can be quickly applied to a PCB, but a lingering problem is the inability to maintain a uniform distribution of the heat over a large area resulting in unacceptable temperature differentials across the PCBs.
The present invention is directed towards a gas convection system for uniformly heating PCBs.
The present invention addresses the above limitations and problems of the prior art techniques. In the present invention a circuit board heater provides a surface face at least as large as a PCB being heated. The surface face is constructed and arranged to emit a heated gas. The non-populated (no components) flat etched side (the component side could also be heated) of the PCB is presented in close proximity to the surface whereby the PCB is heated.
The heat emitting surface face is preferably a surface of open-cell metal foam. Heated gas is introduced into the metal foam and is forced through the foam, diffusing uniformly to the surface face and therefrom to the PCB. In another embodiment, a woven fine mesh metal fabric overlays the heat emitting surface face. This fabric acts as a final gas diffuser and blender in addition to providing additional backpressure to the gas flow. This backpressure aids the uniformity of the emitting gas, as is well known in the field.
In one preferred embodiment, an electric current carrying heating screen is placed between insulating layers of ceramic foam that allow gas to pass through. The screen is porous to gas and preferably covers and provides uniform heating over the entire face opposite the surface face adjacent to the PCB. Gas is introduced through the screen and is heated thereby, and thereafter travels through the foam diffuser to the heat emitting face.
In another embodiment, a gas is fed into one or many conduits that run throughout the metal foam. Each conduit has a plurality of gas outlets. The gas is forced into the conduits and exits the gas outlets of the conduit into an open cell metal foam gas diffuser. The diffuser accepts the heated gas and transports that gas to the heated gas emitting surface face, through a mesh metal fabric, and therefrom to the PCB. The gas may be heated before entering the conduit in a preferred embodiment, or preferably the inner surface of the conduit is heated by an external power source and the gas is heated while travelling along the conduit. In a preferred embodiment the conduit itself carries an electric current and the electrical resistance of the conduit is designed along with the magnitude of the current to heat the inner surface of the conduits to a designed temperature. The gas coming into contact with the inner surfaces is heated.
In another preferred embodiment the conduits may be designed to achieve a temperature gradient along the conduits and the gas emitting openings in the conduits may be of differing sizes and spacings to allow uniform dispersion of the heated gas.